Managing early spring insects in nurseries and Christmas tree fields

With the recent warm weather, growers should be on the lookout for several early insect pests such as white pine weevil and Zimmerman pine moth.

White pine weevil adults mating. Notice very small punctures on the leader and sap droplets associated with spring feeding and egg deposition. Photo credit: Daniel Herms, OSU, Bugwood.org

White pine weevil adults mating. Notice very small punctures on the leader and sap droplets associated with spring feeding and egg deposition. Photo credit: Daniel Herms, OSU, Bugwood.org

White pine weevil and Zimmerman pine shoot moth may become active with the recent warm weather. Looking at growing degree day (GDD) information on Michigan State University’s Enviro-weather, counties in the southern half of the Lower Peninsula are at or will be at 25-50 GDD base 50 by this weekend, March 12-13, 2016.

During these first warm days, overwintering adults of white pine weevil emerge from the duff under the trees and become active between 25-220 GDD50. Zimmerman pine shoot moth larvae also begin leaving their overwinter site at 25-100 GDD50.

White pine weevil

Over-wintering white pine weevil adults move from their overwintering sites and climb up the tree trunk to the leader where they mate and lay eggs. Females may also fly to other trees. The adult female weevil carves out a small, round hole in which she lays one or two eggs. You may see a little resin oozing out of these feeding and egglaying holes, but otherwise may not notice the feeding. Over a few weeks, a single weevil may lay up to 200 eggs on the terminal. The eggs hatch within a couple of weeks and larvae will bore into the terminal to just under the bark to feed in the cambium area.

There are a couple of methods MSU Extension recommends to detect when adults begin to emerge in spring. Research in Pennsylvania has shown that emergence begins once soil temperatures in the top 2 inches were above 50 degrees Fahrenheit. Check out the data from your nearest MSU Enviro-weather station to see what the soil temperatures in your area have been or purchase a soil temperature probe for use on your farm.

Growers may also try using tedder traps to detect white pine weevil adult activity. Historically, we have trapped the first weevils in Michigan around 35 GDD50. Place tedder traps next to trees that were damaged the previous year. Weevils over-wintering at the tree bases are attracted by the scents of alcohol and turpentine that mimic the odor of pine trees. Tedder traps can also be purchased through Great Lakes IPM in Vestaburg, Michigan.

Zimmerman pine moth

Zimmerman pine moth is another insect that can be a real problem if you have Scots and Austrian pine. This insect overwinters as a tiny caterpillar and bores under the bark early in the spring. The larvae will tunnel under the bark for several weeks during the summer causing large, soft masses of pitch to flow from the tree.

Zimmerman pine moth larvae usually bore into large branches or, more commonly, into the stem of the tree, often right at the branch whorls. The tunneling can kill branches and tree stems may break off above the wound. Trees attacked the previous year were more likely to be attacked again. This means you need to look for heavily infested individual trees. Cut and destroy those trees by chipping or burning them as early in the season as possible. That should help remove the most attractive trees from the field and will kill the developing larvae.

Zimmerman pine shoot moth

Pitch mass from Zimmerman pine shoot moth on the trunk of an Austrian pine. Photo credit: Rayanne Lehman, Pennsylvania Dept. of Agriculture, Bugwood.org

Management

Controlling these insects may involve applying a registered pesticide early in spring. The weather forecast is for warm temperatures through the middle of next week followed by cooler more average temperatures. Years with a quick early spring warm-up followed by average temperatures later in March can be a challenge for timing insecticide applications to control these pests. Growers have two options: either make an insecticide application now and then again in a couple of weeks, or wait for the next warm break in temperature to apply your first insecticide application. With white pine weevil, you will want to make another application when the terminal growth begins. What you decide to do with your first application will depend on the amount of damage you have and if you want to accept some risk.

In addition, make sure to thoroughly cover the leader and the upper part of the tree for white pine weevil. You can also reduce future populations by removing dead and dying leaders that contain the developing larvae in June, effectively interrupting their life cycle. Dead leaders should be removed from the field and burned or buried to prevent infestation.

To control Zimmerman pine moth, the insecticide must be present on the bark as the caterpillar bores in, otherwise it will be well protected under the bark for the rest of its life cycle and not susceptible to pesticide application. For the insecticide application to be effective, good coverage of the tree trunk is essential.

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